Why Doesn’t My Air Conditioner Shut Off?

When the weather is extremely hot, we often get requests for service because the system is running all of the time and won’t shut off.

However, when we check the system, we find nothing wrong with the unit. It is simply doing all it can to keep up, so we thought we would explain why this happens.

When sizing the cooling load for a home, contractors use Manual J. This references design temperatures to use to determine what the demand will be 99% of the time spent in cooling months using a 30-year average. With that data, they size the equipment to cool 20 degrees below the outdoor temperature.

The design temperature for our area, using data compiled from Zanesville in 2017 (the most recent data available), is 87.4 degrees. 20 degrees below that is 67.4. That means when we reach outside temperatures above 87.4, which occurs only 1% of the time out of the total cooling hours, our system begins to have to run longer and longer and may not shut off when our area reaches 90 degrees and higher extremes.

The equipment is sized for the design temperatures, not the rare extreme temperatures. Why, you may wonder? If equipment is oversized, then under normal design temperatures, it will not run long enough to dehumidify, leading to a clammy type atmosphere, and comfort is compromised.

Most systems are on full blast or off. However, there is variable speed equipment available which ramps up or down gradually according to demand. You would need to decide whether the increased cost of that type of equipment is a sufficient trade-off for increased comfort.